In malignant brain tumors, cancer stem cells (CSCs) have recently been defined in functional assays of self-renewal and tumor initiation. Though often only present in very small numbers in human tumors, CSCs have the ability to generate tumors that recapitulate the original tumor when xenotransplanted into animals, whereas the remaining non-CSC tumor bulk most often cannot. Thus, elucidating molecular targets critical to CSC biologies may generate strategic therapies which improve patient survival. Though expressed in a variety of cancers, including malignant brain cancers, the role of inducible nitric oxide synthase (iNOS) in tumors is unclear, since several pro- and anti-tumorigenic roles of NO and iNOS have been delineated in various systems. Our preliminary evidence suggests that iNOS expression is largely limited to the CSC population in a variety of malignant brain cancers, expression is induced in response to radiation in the CSC population, and iNOS activity may be crucial to the survival of CSCs at baseline and during radiation therapy. As a result, we hypothesize that iNOS in brain tumor CSCs contributes to tumor malignancy and radioresistance and targeting iNOS in CSCs will provide valuable therapeutic approaches to the treatment of brain tumors. To investigate this, we will pursue the following specific aims: 1) determine the differential role of iNOS in normal neural stem cell and brain CSC behaviors, 2) determine the role of iNOS activity in brain tumor CSC radioresistance, and 3) determine if pharmacologically targeting iNOS can decrease the tumorigenesis and radioresistance of brain tumor CSCs. To do this, we will employ both shRNA knockdown technology and a specific iNOS inhibitor to investigate the role of iNOS in cancer stem cell maintenance phenotypes, post-radiation survival, and in vivo tumorigenesis and tumor radiation resistance. The long term goal of this project is to develop cancer stem cell specific therapies to increase brain tumor patient survival. The overall goal for my training is to develop the skills necessary to become a physician-scientist dedicated to improving neurologic patient care through linking research in the laboratory and clinic. PUBLIC HEALTH RELEVANCE: Brain tumors are extremely deadly and reappear frequently after treatment, resulting in patient death. Some cells in a tumor are especially resistant to radiation therapy and probably help tumors reappear afterward. We aim to investigate the ways these cells resist radiation and will investigate possible treatments able to prevent these cells from regenerating tumors. We expect this approach may generate new and effective treatment strategies for the tens of thousands of patients diagnosed with malignant brain tumors every year.